This invention relates to glass ceramic composite materials and more particularly relates to reinforced cordierite type glass ceramics.
It has been known in the prior art, e.g. from U.S. Pat. Nos. 2,920,971; 4,070,198 and 4,415,672, that glass ceramics could be obtained through the controlled crystallization of a glass body to form relatively uniformly sized, fine grained crystals homogeneously dispersed throughout a residual glassy matrix. The residual glassy matrix will generally have a composition different from that of the precursor glass body since components comprising the crystal phase will have been removed from the glass.
Such glass ceramics therefore have properties which are radically different than the precursor glass material. Such glass ceramics may for example have improved strengths, higher temperature capabilities, improved resistance to thermal shock and altered, often lower, coefficients of thermal expansion. Despite the elapse of several decades since the discovery of glass ceramics, improved glass ceramics are still constantly being sought by trial and error. This is due to the continually increasing requirements for high performance materials as a result of advancing technology and is also due to the difficulty of predicting the properties of a glass ceramic prior to its development and testing. In a particular application, to protect electronic parts in a high temperature environment, e.g. in a high velocity atmospheric vehicle, the ceramic, in addition to high temperature resistance, must also retain good strength, have a high degree of microwave transparency and desirably have good thermal shock resistance. A glass ceramic which has been used in these applications is described in U.S. Pat. No. 2,920,971. While this material, which comprises MgO, Al.sub.2 O.sub.3, SiO.sub.2 and TiO.sub.2, is adequate for present applications, future requirements for similar applications will demand greater refractoriness and higher temperature strength, e.g. &gt;1200.degree. C., greater thermal shock resistance, and minimum change of a low dielectric constant over a broad range of temperatures and frequencies while maintaining microwave transparency. Attempts to reinforce these older ceramic materials with ceramic fibers or crystalline whiskers have not been very successful since one or more properties are adversely affected, often by lack of adhesion of the glass ceramic to the reinforcing material for reasons not clearly understood, see e.g. "Development of Broadband Radome Material", Laden et al, July 1982, United Technologies Research Center Report AFWAL-TR-82-4100.
A nitrogen containing glass material has been developed which can be converted to a glass ceramic material without the presence of usual crystal nucleating agents, such as TiO.sub.2, ZrO.sub.2 or SnO.sub.2. The materials, e.g. as specifically described in U.S. Pat. Nos. 4,070,198 and 4,097,295, still lack some of the physical properties desired for the newer requirements.